i have never seen a need to switch to serenity if your single target healing you should be disc otherwise its a holy shit moment and blowing 2 globals for a crap instant over 2 flash and a serendipity greater would benefit more. Comparing the Sanc spell to the serenity spell is apples and oranges the real difference is the bonus effects and sanc is def a load better.

I would like to point out that the real benefit of Serenity isn't so much the slightly better single target healing, it's the rolling Renew mechanic. An example would be hardmode Conclave of the Wind, when you are stranded on blue with the tank. It's easy to keep Renew rolling on both of you which you can keep up with a mana neutral Heal or Serenity cast. If you both take enough damage, a quick Binding Heal will refresh both Renews. By all means, you could just sit in Sanctuary and throw out the same heals, but I prefer to save my mana for when I actually need it.

It's still very situational though. I find Sanctuary to be better most of the time like some of the other posts here, but if Serenity is a good fit for a particular phase you might as well use it.

Disc is better in 10m and holy is better in 25m generally speaking. Numbers will varry based on fights, player skill and healer setup.

Since when holy is better than disc for 25 man? Or other question. If there is not any other to have holy and discipline priest at once in raid I think much better is discipline. Heroic raiding of course.

For my $0.02, I don't play a priest but I do lead a heroic 10 raid with one (as well as a druid sham and pally) and I always experiment with comps to figure out the optimal setup for most fights. Optimal spec varies based on fight, but as a general rule if you're assigned to tank healing on a fight go for Disc. Holy does way worse at this job.

I prefer to have our priest go:
Disc for Halfus, Cho'gal, Conclave (on air boss), Maloriak, and Nef
Holy for VnT, Al'akir, Chimeron,
Either spec for: Magmaw, Omno, Atra - depending on how much tank healing we need in the comp
Have not done Elemental C or Sinestra yet

That's just what's worked best for us, it does also vary depending on skill or lack thereof with each spec and on the rest of the comp. Keep in mind the other classes also have tankheal builds and raidheal builds which change the comp.